Typically, a steel belt for a pneumatic tire means a belt formed by steel cord of several twisted filaments on which a layer of thin rubber is overlaid. In general, steel cord is produced through the process of drawing carbon steel with a carbon content of 0.70.about.0.82% by weight, plating brass on the surface of the carbon steel so that it adheres to the rubber and then drawing again and stranding said carbon steel by various methods.
The use of carbon steel with carbon content of more than 0.70.about.0.82% by weight results in high strength. However, many problems have arisen with respect to manufacturing wire rod for steel cord because if the content of carbon is increased, then when manufacturing wire rod, it is difficult to control carbon segregation and the distribution or the size etc. of the nonmetallic inclusions; moreover, if there is much carbon segregation or the size of nonmetallic inclusions is not well controlled in the carbon steel, when manufacturing wire rod for steel cord, then these cause manufacturing defects, such as the problem of cutting during drawing, since it is hard to endure the heavy drawing process during the production of steel cord for tires, and accordingly the production of steel cord is difficult.
U.S. Pat. No. 5,061,557 discloses a cord of two twisted filaments made from steel with a carbon content of 0.70.about.0.90% by weight. Even though the strength of the cord can be increased partly by appling steel with a carbon content of 0.80.about.0.90% by weight, it is not enough yet.